The present invention relates to a process for extruding a plastic foil with small variations in thickness and, more particularly, foils requiring a highly constant thickness such as, for example, foil or films of laminated glass.
Laminated glass film made from Polyvinylbutyral and plasticizer is usually produced in such a way that the raw material is melted in a worm or spiral press, and the melted mass is then introduced under pressure from the press into a tool having a broad slot. This broad slot tool thereafter carries out the deformation of the melted mass into the desired breadth, so that at the end of the production sequence the melted mass leaves the slot tool in a broad path or course and is subsequently cooled off by means of after-connected units.
Even with temporary or transient constant dosages of the raw material -- whereby polyvinylbutyral and the plasticizer in a permanent proportion, separate or mixed but preferably mixed, are introduced by means of appropriate dosage devices -- the variations in thickness of the film produced thereby are still relatively large and are approximately 0.06 to 0.08 mm. These variations, however, are too large to produce an optically faultless laminated glass which utilizes the film. It is desirable to keep the variations in thickness within much more narrow limits.
With regard to the conventional method of producing such film, marginal strips occurring during the cutting of the film by means of subsequent devices or, for that matter, waste products can be introduced into the worm press, whereby the marginal strips and waste products are chopped directly in front of the inlet into the filling funnel or bin and added as shreds or slices.
In the past these marginal strips and waste products have been added unmeasured so that minor variations in quantity per unit of time cannot be avoided and thus the danger of variations in thickness of the extruded film is especially high.
The variations in thickness of the extruded film can be kept small, if between the worm press and the tool a melting pump is connected--the efficiency and field of operation of which are known--having a constant output per unit of time at a constant speed of rotation. Due to the constant output of the melting pump, however, a worm press equipped with a principally constant dosage would, in the course of time, change its filling degree, which is the proportion of the space used by the melting mass towards the entire space being available within the worm press, because the output of the melting pump and the output of the measuring devices (dosage devices) of the raw material cannot practically be tuned in or coordinated to each other in the long run. In one extreme case, the worm press would be overfed while, in the other extreme case, it would empty to an extent where it would be underfed. In neither cases could a film of constant thickness be extruded, especially since adding the marginal strips and waste products would change the quantity so quickly that the extrusion process under constant conditions would not be possible at all.